1. Field of the Invention
The present invention generally relates to a method of measuring a concentration of the fuel, and more particularly a simple and high-accuracy method of measuring the concentration of the fuel.
2. Description of Related Art
With the progress of industry, consumption of traditional energy sources, such as coal, petroleum, and natural gas, are continuously growing. Due to the limited reserves of natural energy, it is necessary to develop new alternative energy sources to replace the traditional energies, and among them, fuel cells are the important and practical one.
In brief, the fuel cells are basically a power generation device that converts chemical energy into electric energy through the inverse reaction of water electrolysis. For example, a proton exchange membrane fuel cell (PEMFC) mainly includes a membrane electrode assembly (MEA) and two electrode plates. The MEA includes a proton exchange membrane, an anode catalyst layer, a cathode catalyst layer, an anode gas diffusion layer (GDL), and a cathode GDL. The anode catalyst layer and the cathode catalyst layer are respectively disposed at two sides of the proton exchange membrane. The anode GDL and the cathode GDL are respectively disposed on the anode catalyst layer and the cathode catalyst layer. Furthermore, the two electrode plates include an anode and a cathode respectively disposed on the anode GDL and the cathode GDL.
Currently, the common proton exchange membrane fuel cell is the direct methanol fuel cell (DMFC), which directly uses a methanol solution as a fuel supply source to generate current through the relevant electrode reaction of methanol and oxygen. The reaction formulae of the direct methanol fuel cells are expressed as follows:Anode: CH3OH+H2O→CO2+6H+6e−Cathode: 3/2O2+6H+6e−→3H2O
During the reaction, the concentration of the methanol solution introduced to the anode has great impact on the stability of the output of the direct methanol fuel cell. If the concentration of the methanol solution introduced to the anode is not properly controlled, disadvantages of poor power generation efficiency and unstable output power may be generated, and further, the MEA may be damaged. Therefore, it has become the important subject in the research and development of the direct methanol fuel cells how to properly supplement methanol so as to control the concentration of the methanol solution introduced into the anode in a suitable range.
The most direct way to control the concentration of the fuel in a fuel cell is to directly measure the concentration of the fuel by a sensor and determine the supplement amount of the fuel and water. This method has been disclosed in TWI 228591, U.S. Pat. No. 6,589,671 B1, U.S. Pat. No. 6,488,837, US 2002/076589 A1, US 2003/0196913 A1, WO 01/35478. U.S. Pat. No. 6,488,837 and US 2003/0196913 A1 have disclosed the MEA serving as a sensor for directly measuring the concentration of methanol. It should be noted that the accuracy of the above method is liable to be affected by the impurities in the fuel, aging or unstability of the MEA.
In the prior art, for example, in U.S. Pat. No. 6,698,278 B2, the measured temperature and the current value are brought into an empirical formula to calculate the concentration of the fuel. This method can directly measure the concentration of the fuel without using a sensor. However, this method must be adjusted according to different fuel electrode systems so as to calculate the concentration of the fuel. U.S. Pat. No. 6,589,679 and TWI 282639 have also disclosed the methods of directly measuring the concentration of the fuel without using a sensor.
Furthermore, since the concentration of the methanol solution is somewhat related to physical properties such as the transmission speed of the sound in the methanol solution, the dielectric constant of the methanol solution and the density of the methanol solution, the transmission speed of the sound in the methanol solution is used to calculate the concentration of the methanol solution in some prior arts. It is noted that TWI 251954 has disclosed that the dielectric constant and the density of the methanol solution can be used to calculate the concentration of the methanol solution. However, the sensor used in this concentration calculating method is expensive and the precision of measurement is significantly affected by bubbles in the methanol solution, the methanol solution inside the sensor must be kept still and have no bubbles when measuring, which increase the difficulties in the measurement.
In view of the above concentration measuring methods, the problems of difficult measurement, high cost, and poor accuracy exist. Therefore, it is urgent to find a simple and high-accuracy method of measuring concentration of the fuel currently in this field.